A window sash mounted for sliding vertical movement within a window case, also known as a double hung window, can require a significant force to raise the window. To equalize the required raising force with the force needed to lower the window, elongated metal coil springs generally disposed laterally adjacent to the opposing sides of the sash and within a vertical channel defined by jamb liners have been provided. One end of the spring is generally attached to a stationary part of the window case or jamb liner, and the other end of the spring is attached to the movable sash or, more typically, to a shoe or other sash support member slidably mounted within a vertical guideway defined by the jamb liners and/or the jambs. Such balance springs are designed or selected so that they urge the window sash upwardly with a force which is generally about equal to the weight of the sash. As a result, the frictional forces between the sash and jamb liners, or between the shoe or other sash support members and the vertical guideways can be minimized, so that both opening and closing the window can be accomplished with very little effort.
A problem with conventional spring balance mechanisms used to balance the weight of a window sash is that, as the window is raised or lowered, the spring vibrates. This vibration causes the sides of the metal spring to rapidly and repeatedly impact against the rigid plastic walls of the vertical channels in which the spring is disposed. Such interactions between the spring and the walls of the vertical channel often create an unpleasant and objectionable clattering or rattling noise.
Attempts to reduce or eliminate the noise caused by vibrating springs have traditionally been limited to attaching flocking to the springs. This involves applying an adhesive to the spring and covering it with tufts of fiber. The flocked springs, however, do not sufficiently reduce the level of noise generated as the sash is raised or lowered. In particular, many people regard the noise generated by flocked springs to be almost as objectionable as the noise generated by a spring which has not been flocked. Further, any reduction achieved by flocking the spring is degraded over time with continued use of the window, since tufts of fiber are gradually removed from the spring by the repeated vibrational interactions between the spring and the walls of the channel in which it is disposed. Of course, the use of flocked springs adds to the cost of the spring balance mechanism and hence to the cost of the window. While it has been proposed to use strips of soft plastic or the like inside the narrow and often restricted guideways in order to provide a bumper between the spring and hard plastic walls of the guideways, that approach involves manufacturing complexity and difficulty, with significant and potentially undue additional expense.